1. Field of the Invention
The present invention relates to metal particle-dispersed composite oxides, metal particle-dispersed composite oxide-sintered bodies, a method of manufacturing metal particle-dispersed composite oxides, and hydrocarbon-based fuel reformers.
2. Description of the Related Art
In recent years, fuel cells are attracting much attention as being a clean and highly efficient power generating technique. As for the method of producing hydrogen gas employed as a fuel for the fuel cells, there is known a so-called “reforming” technique, wherein hydrogen gas is derived through a reaction between hydrocarbon-based fuel and water vapor, etc. by using catalysts or reforming materials. As for the reforming materials, there is generally employed a structure comprising an oxide ceramic carrier formed mainly of alumina, magnesia, silica, etc. and carrying on the surface thereof fine particles of a noble metal or fine particles of an active metal such as Cu, Ni, Co, etc. In order to enable the reforming materials to be successfully employed in a catalyst system (formed of a catalyst and a carrier) for this reforming, it is imperative for the reforming materials to have the characteristics that metal particles functioning as a catalyst are uniformly dispersed throughout the surface of a carrier carrying the catalyst, that metal particles are strongly bonded onto the carrier so that the catalyst particles are prevented from being desorbed during the reforming operation, and that the reforming materials are hardly deteriorated even in the usage thereof for a long time.
For example, although Cu-based catalysts which are employed as a catalyst for steam reforming of methanol are excellent in activity as well as in selectivity, the Cu-based catalysts are poor in stability and not suited for use in an operation taking a long time. On the other hand, noble metal-based catalysts are excellent in stability, but are expensive and the resources thereof are limited. Under the circumstances, catalysts comprising Ni or Fe are now being studied for use as a prospective material for a cheap and stable catalyst.
These metal particles employed as a catalyst are generally applied onto the surface of a carrier formed of ceramics, etc. by a co-precipitation method, etc. According to this method, fine particles comprising a catalyst element are precipitated on the surface of a catalyst carrier and then, reduced to obtain a catalyst system where catalyst particles formed of metal particles are dispersed the entire surface of the carrier. According to this method, the particle diameter of the catalytic metal can be made very small, in the order of nanometers. However, the co-precipitation method is accompanied with several problems, in that it is difficult to control the state of the dispersion of catalyst, thereby giving rise to the growth of metal particles in a heated environment, and that the bonding strength between the catalyst and the carrier is relatively poor, thereby desorbing the catalyst during the use thereof for the reforming. Furthermore, since porous pellets are employed as a carrier to be charged into a reactor, there is a problem that it will lead to an increase in pressure loss, thereby making it necessary to feed the hydrocarbon-based fuel to the reactor while applying a pressure to the fuel.
Meanwhile, in recent years, the technique for a small reactor having fine channels provided on a flat substrate with the aforementioned catalyst being positioned on and along these channels, wherein liquid hydrocarbon fuel such as methanol, ethanol, etc. is employed, is attracting much attention as being useful for a micro-reactor, etc. In this case also, the catalyst is generally applied onto the surface of a carrier by the co-precipitation method, etc. However, this technique is also accompanied with problems that, in addition to the problems mentioned above, it is difficult to uniformly precipitate the catalyst on the surface of the fine channels on the occasion of enabling the fine channels to carry the catalyst thereon, thereby flocculating the catalytic particles mainly at the corner portions of the sidewalls of the channels. As explained above, these conventional methods are still incapable of developing a fuel reformer having features which are fully satisfactory in terms of life and activity.
On the other hand, there is reported a method of preparing a material wherein a ceramic solid solution phase such as a Ni—Mg—O system or a Fe—Al—O system is subjected to a reducing treatment in a reduction atmosphere so as to precipitate metal particles such as Ni particles or Fe particles. Since metal particles in these materials are enabled to precipitate from the interior of ceramics through a solid-phase reaction thereof, the compatibility of the metal particles with the ceramic phase functioning a base phase is excellent and hence the metal particles are excellent in dispersibility. Further, since the metal particles precipitated in this manner are formed of a material of the same kinds as transition metals-based catalysts such as Ni-based catalyst, the metal particles are expected to be useful as a catalytic material. As a matter of fact however, since the materials manufactured in this manner are featured such that the metal particles are excessively precipitated in the interior of the ceramics, it has been considered difficult to utilize them as a catalyst.
If the metal particles are to be utilized as a catalyst, it is preferable that the metal particles exist only on a surface region of a composite oxide. Further, as far as the activity of catalyst is concerned, the number density of the metal particles (the number of catalytic particles per unit area) should preferably be as high as possible, and also the specific surface area of metal which is advantageous to the catalytic reaction should preferably be as large as possible. Therefore, if it is possible, in the aforementioned metal precipitation method utilizing a reduction treatment, to promote the precipitation of metal particles at the surface of a composite oxide while suppressing the precipitation of the metal particles at the interior of the composite oxide, the catalytic properties of the metal particles are expected to be further enhanced.